Another "Bex Solution" by cheapracer, because Bex is Better! 4 links trailing arms should not be parallel!! .... even though 99% of the mod car population thinks that it goes without saying they should be. Trailing arms do not roll around a center point between them, nor do they rise in a 2 dimensional plane in roll, they roll 3 dimensionally controlled by points (RC and chassis pivot) some distance away, there's 2 sides of them and they are at different points of that arc hence their longitudinal chord lengths change at different rates. This is pyhsics that can't be changed regardless that your brain is now screaming "Trailing links should be parallel!!". Why do links bind? Because they immediately become non-parallel (longitudinally) at the first instance of roll and the change in the link's chord lengths opposes the opposite side's trailing links that are also trying to do their thing causing bind.. specifically the top link in bump shortens faster than the opposite side's top link in droop and the axle won't twist to compensate. By having non-parallel links we have one side's top link chord length shorten in roll (bump) while the opposite side's top link shortens at closer to the same rate while in droop. The test setup; For a practical sample I chose the very popular Locost 4 link rear trailing link setup as per the book specifications. Although using some scrap steel, great care was taken in dimensional accuracy. Note horizontal heims were used on the dummy "axle" upper pivots to enable quick height adjustment for the test .. Links and jig; Links were 292mm in length as per book, initially 140mm vertical spread and all 4 jigged for accurate length .. The result; With the links parallel as per the book specs and common mythology that 4 links should be parallel(not to mention 99% practical application), bind was quickly established at around 6 degrees. Axle had to be mildly forced to this limit, would not fall under it's own weight after about 4 degrees and binding was felt well before limit. Note at this point all 4 links were very tight to swivel due to the binding and had not reached their travel limit .... With the links set non-parallel (rear pivots set 8mm closer together), bind was non-existent, the axle fell under it's own weight until travel limit of horizontal heims was reached. note that the only reason the axle stopped at this point was because the horizontal heims ran out of travel - THE AXLE WOULD HAVE TRAVELED MUCH FURTHER FREELY with vertical heims. The lower links swivelled freely and easily indicating little bind. Stupid picture should read "255mm" on the left.. The resulting "winning" dimensions that you might use on your Locost, note the horizontal hiems have run out of travel .... I also tested upside down with the RC centered to a live axle (above test with RC 50mm below wheel centerline) with similar results... note in the lower picture the trailing arms have reached horizontal having traveled further and again i stress that it only stopped there because the horizontal hiems ran out of travel, not because of bind! I take these tests very seriously and was very careful to check and recheck before taking results. Should you do it even though theres thousands of Locost's running around today successfully? Well that's entirely up to you now you have the information, note that the serious binding starts near the end of suspension travel limits on a typical Locost but it is there. At maximum bump this will tend to creat oversteer due to the binding creating anti-roll besides the higher wear/stress on your heims and chassis mount stress. Of course rubber bushings masks this (but doesn't remove the binding anti-roll) but that's not good enough for me and it shouldn't be for you. On a Locost this should be very easy to check/modify for yourself by merely adding 2 extra holes on the rear axle brackets as shown (can weld a washer over later) or raise the front mounts the same amount. Note that at 120mm test seperation binding had seriously returned, 132mm was the sweet spot; As another proof of point, I currently have a mock up of another 4 link (DeDion) with 710mm long non-parallel trailing arms with vertical seperation of the pivots at 310mm front and 290mm rear - a full 20mm difference. With the trailing arms parallel I get serious bind within 100mm of travel but with the rear pivots 20mm closer I can lift either side a full 200mm freely using 1 finger and only limited by chassis interference in this case. Pictures another day, phone battery dead. Don't believe me? Make up your own test rig, use some wood, nails and hook screws or try this on your current 4 link, it's not that difficult to prove or understand once "you see it"! Some people will not be convinced because of the mindset about what 4 links should be ie; "Parallel!!" but a few of you more flexible thinking might benefit from this for your builds ;-)
Sounds cool, but since I'm not an engineer I'm easy to impress. Non-parallel designs are used on all kinds of race cars, I believe. Waiting for the second shoe to fall... Gary
Does that also mean that if mounts on the frame end where widend 8mm it would have the same effect? Does it matter how long the arms are or odes this not change anything?
Actually a lot use parallel, complain about the bind and then switch over to 3 link. 3 link is not better than a good 4 link but resolves the problem that they had with a bad 4 link so they believe it is This is a quote from a seasoned racer; "Cheapy, believe me they are bound up. If you use 2 lower links and 1 top one the car rolls. Use 2 top links and the car lifts inside wheels. As an aside my Sports Sedan used a 3 link and panhard. And it put the power down better than most because the suspension was not trying to pick up the inside wheels. The car could roll and keep both tyres flat on the ground". It's not that people are ignorant, just some mindsets are set in stone, apparently, and will not be wavered from. There's many, many examples, Google "4 link bind" ...
Don't think so because you're not changing the longitudinal chord length nor it's arc by much Yes, apparently it matters, I gave examples above where 8mm works for Locost book build and 20mm works for mine. I don't know if this is because of length and separation or more specific to just length. Note that my trailing arms are just over 2 times longer and my separation reduction is also just over 2 times greater so that would offer the statistic of 10mm for every 300mm - wouldn't bet my life on that but may be a rough starting point ... A friend has already asked me to check his design, I'll report back when I do.
I'm sure I can construct a geometric proof of the conditions under which multiple parallel links won't bind, but I'm a bit out of it at the moment. In any event, I'm subscribing.
and this isnt even talkin about how a reverse 4 link is dangerous and stupid. thats a whole other thread
I think that moving the links closer together at the axle might move the instant center to an unfavorable location. I'll have to check it with the analyzer program later, but I believe it moves the instant center to the rear of the car behind the axle. Was the binding on bump with the parallel bars caused by lack of range of motion on the heims?
According to text, the binding was from the horizontal mounting of the test heims. Vertical would have more free movement.
I think because of Watson Roadsters in early Indy. Some were front wheel drive. 'Golden Eagle' presides.
mostly minitruckers but theres a biuld thread on here that a dude bought a s10 that was bagged to put under his chevy pick up and it has a reverse 4linlk wherein AZ are you todd?
My goodness 4 link rear suspension. The leading radius rods angle wil determine the instant/center & is the best benefit of this type of suspension. In almost all racing aplications the top bar is pointed down to establish the instant center for best accelerating behavior. There were as many suspension link ideas as there were car builders when solid axle cars were the standard. Now days about the only cars left in racing that use solid axels are sprint & midget cars. On the street there is not as big a requirement for these links to be set up aggresivly as in race cars. Also by using a anti roll bar the links being parralel will be even less important.
My OT/DD came from the factory with a reverse 4-link, on the front. Worked well. It has radius arms now.
This mod has no bearing at all on lateral location, in fact exactly the opposite as the whole range of travel will now be bind free and have no effect like a binding system does. No, excerpt from the main post above; Why do links bind? Because they immediately become non-parallel (longitudinally) at the first instance of roll and the change in the link's chord lengths opposes the opposite side's trailing links that are also trying to do their thing causing bind.. specifically the top link in bump shortens faster than the opposite side's top link in droop and the axle won't twist to compensate. and; Note at this point all 4 links were very tight to swivel due to the binding and had not reached their travel limit .... You could also go to this thread for further reading .. http://www.jalopyjournal.com/forum/showthread.php?t=705574 I'm guessing you are refering to a birdcage setup. Indeed, look up Mallock race cars for example Ummm, no. Masking something doesn't make it right, and then there's those pesky things called bumps... Marcos Cars came with it standard.
How are the reverse 4-links you're talking about done that makes them dangerous and stupid? In other words, how does that condition result in them being dangerous and stupid? In other words, what are the conditions under which they are dangerous and stupid? In other words, what, if any, are the conditions under which they are neither dangerous nor stupid? Answer that, and we'll all have learned something more useful than that you don't like reverse 4-links, which you must admit doesn't really shed a lot of light on the subject. Off the top of my head I think there might be something worth exploring in the potential for self-correction in this sort of tensile geometry.
Hi Cheapracer, saw your post on parallel links and the nice test rig you made up. this helps illustrate the working of a 4 bar and the operation but I have some questions. First since the test bar length is so short it seems to exaggerate the problem. However in each case you have taken either the upper or the lower control bar and limited the rotation by indexing either one end upper or lower heim end 90* to the operation of the bar thus limiting the travel to the missalignment angle of the ball end and producing a bind. All ends must operate in the same plane, operating one set at 90* to the opposite side just turns them into misalignment rods. Redo your test over with the heim ends all indexed on the vertical plane and note the change.
You did not install a track rod (diagonal link) so what happens when you do? Also, your axle width is very narrow, set it out to 57" and redo the tests. Joe
I have no idea why you figure that as hiems don't care if they are 0, 38, 90 or 143.56748 degrees. It's just a ball. The only issue is of course that you run out of movement quickly in the horizontal orientation whereas vertical orientation can turn 360 degrees. Anyway, above I mentioned; As another proof of point, I currently have a mock up of another 4 link (DeDion) with 710mm long non-parallel trailing arms with vertical seperation of the pivots at 310mm front and 290mm rear - a full 20mm difference. With the trailing arms parallel I get serious bind within 100mm of travel but with the rear pivots 20mm closer ... ..here is a video of me running it through 10" of movement with 2 fingers and totally free movement - to make matters worse the opposite side is even at full droop and yet ..
The video above is 52" and is lateraly located - that should be clear actually as it obviously goes up and down on the same path. I can flop the test rig around side to side, up and down and any angle within those parameters freely without any restriction, I'm happy
it makes a difference as with all the ends phased one way you get 360 degrees of rotation, if they are phased 90 degrees you only get 24 degrees of rotation without the correct spacers.
Thanks, I actually thought that was quite well established. i want to know what Dick mean't by "mis-alignment"...? 24 degrees? Depends on the brand and the brand model. With the increasing popularity of rock-crawlers, 30 degree hiems are becoming more common.
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